Distribution of curve flexibility in AIS: a multicenter study of 5,267 patients

Curve flexibility plays an important role in the treatment of adolescent idiopathic scoliosis (AIS). This study aimed to describe the patterns and variability of curve flexibility in a large patient cohort.

Five thousand two hundred sixty-seven surgical patients from an international AIS registry were analyzed. Curve flexibility was calculated as percent change from the major Cobb angle on bending radiographs. Associations between curve flexibility and demographic, clinical, and radiographic parameters were assessed using non-parametric tests and linear correlations.

Thoracic curves were less flexible than lumbar curves (35.5% vs. 49.2%, p < 0.001). Thoracic flexibility decreased by Lenke type: Lenke type 1 (40.4%), Lenke 2 (32.9%), Lenke 3 (26.8%), and Lenke 4 (19.3%) (p < 0.001). Lenke lumbar and sagittal modifiers further influenced thoracic flexibility with lumbar "C" modifiers (p < 0.001) and hypokyphotic "-" modifiers (p < 0.001) having the greatest flexibility. Lumbar curve flexibility also varied by Lenke subtype: Lenke 3 (39.2%), Lenke 4 (37.1%), Lenke 5 (56.1%), and Lenke 6 (44.3%) (p < 0.001). Flexibility showed negative correlations with BMI (p = 0.020), Cobb angle (p < 0.001), and deformity angle ratio (DAR) (p < 0.001). Open triradiate cartilage was associated with slightly greater thoracic flexibility (38.4% vs. 35.5%, p = 0.007). Female sex correlated with increased lumbar flexibility (51.7% vs. 43.9%, p < 0.001).

This large cohort provides detailed insight into the distribution of curve flexibility in AIS patients, highlighting inherent biomechanical differences among curve types and patient demographics.

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